Fiber Reinforced Filter Housings

Thirty years ago saw the beginning of fiber reinforced plastic (FRP) pressure vessels and tanks. These were made the same way as today­p;­p;layering fiberglass or carbon fibers with various resins.

The most commonly used corrosion-resistant resins were those of the polyester family. In extremely corrosive chemical environments, a stronger, more corrosion-resistant resin was needed; one that would stand up to not only highly corrosive materials such as hydrochloric acid and sulfuric acid but also resist the high stress and cyclic loading placed on the equipment.

Dow Chemical hypothesized that the addition of an epoxy backbone would greatly increase the resistance dilemma. The strength and toughness of the epoxy would make the resin highly resistant to chemicals and solvents. Then, to enhance the reactivity of the resin, vinyl acrylate groups could be placed onto this epoxy backbone. The result, Dow 411-45 vinyl ester resin, which gives broader corrosion and solvent resistance than older established premium polyester resins. It has been designed for 10 to 20 percent more laminate strength, two to four times better adhesion and three to five times greater elongation than other resins.

If the laminate layer of a polyester resin cracks, chemicals can seep in through the corrosion barrier and destroy the laminate. Without going into a detailed chemical lecture, the vinyl groups of polyester resins are dispersed throughout a molecular chain. This results in cross-linking and brittleness. Derakane 411 resins confine these groups to the ends of the molecules, which allows the entire length of the molecular chain to elongate under stress and absorb mechanical and thermal stress or shock. This inherent resistance to cracking means three things:

1. when properly designed for a given load, the wall of an FRP filter vessel can be made thinner;

2. when designed with a tensile strength of 12,000 PSI and a 10:1 safety allowance, a laminate made with Derakane 411-45 resin shows an actual safety factor of 16:1 or greater while polyester resin laminates have a safety factor of less than 7:1; and

3. when designed to an allowable strain of 0.1 percent, the laminate made with Derakane 411-45 resin has a true safety allowance of greater than 11:1 compared to 5:1 for the polyester resin.

Filament reinforced plastic is made with a reinforcement of either fiberglass or carbon fibers. The reinforcement provides the finished product with the strength and structure while the resin acts as a corrosion barrier preventing corrosive elements from penetrating the FRP material. FRP materials can be stronger than steel on an equal weight basis (termed "structural efficiency" or "strength-to-weight" ratio).

Optimal strength can be realized through proper orientation of the glass reinforcement. During the construction of pipe, tanks or pressure vessels, filament winding is an automated process that yields a hollow shape. Typically, the filament winding process involves continuous glass rovings passing through a tensioner and into a resin bath. From here the saturated glass strands are wound onto a rotating mandrel at a precisely predetermined angle and speed. The filaments are collimated and aligned by a proprietary process that resists both longitudinal and circumferential stresses imposed by the vessels internal pressure. This patented process assures dimensional control on interior sealing diameters when the vessel is subjected to a wide range of both pressure and temperature loads. Filtration vessels constructed of Derakane 411-45 have excellent resistance to corrosion by many different chemicals, including both acids and alkalis. This resistance extends from below ambient conditions to elevated temperatures. FRP vessels require little, if any, maintenance during the course of a long service life.

In addition to the corrosion resistance for the interior of the filter vessel, filters can be designed with long exterior filter life in mind. While there is no direct fluid contact with the externals of the housing, every aspect should be well thought out. For example, anodized 6061-T651 aluminum and 303 stainless steel can be used for standard housings and for special applications, all of the supporting external components may be ordered in 316 stainless steel. The steel is certified per ASME and ASTM Codes and has no welds or weld repairs. Even though there are no metallic wetted parts, this provides external materials free from mercury and/or other foreign contaminants. However, this is not always the case with welded steel housings.

The external fiberglass may be painted with a modified oil alkyd enamel which has excellent resistance to weather, salt and sun. In the case of exceptionally aggressive chemicals, the interior of the filter may be lined with either polypropylene or PVDF. These liners are bonded to the fiberglass to form one integral component.

One other available option is the addition of Benzoyl Peroxide (BPO)
to the Derakane resin. This high boiling point plasticizer aids in curing the resin and halts runaway decomposition. This application is especially useful in sodium hypochlorite (bleach) applications.

Another difference to consider is the type of o-ring seal used. The designed sealing mechanism is a side piston seal similar to that used in automobile engines. This method of construction actually increases the seal compression as the pressure in the vessel increases. The vessel is sealed when the top seal plate is put in place. There is no need to torque the cover bolts of the filter to ensure integrity. Merely hand tightening them is sufficient to provide a leak proof housing. The lower plate of the filter vessel seals with two o-rings to both prevent the housing from leaking and to maintain the integrity of the clean and dirty sides of flow. Due to the flexibility of the design, filter vessels can utilize either double or single open ended cartridges. Either style will provide you with a no-bypass filter housing with a minimal pressure drop. Along with being highly corrosion resistant to a wide range of chemicals, Derakane 411-45 resins will comply with the U.S. Food, Drug and Cosmetic Act, as amended and applicable FDA regulations (21 CFR 177.2120). These resins may be used as articles or components of articles intended for repeated use in contact with foodstuffs.

In summary, the sample process listed above is designed to provide the customer a filter housing with unparalleled corrosion resistance, safety and longevity.

Tom Boisseranc is with Eden Equipment Company, Inc., which designs and manufactures fiberglass bag and cartridge filtration systems where corrosion and chemical compatibility are required. Tom has a Bachelors of Science degree from California State Polytechnic University and has been involved in the filtration industry for nine years. The company may be reached at 8-00-842-5081 or visit www.edenexcel.com.

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